The present invention relates to an image processing apparatus used in an image forming apparatus such as a digital copying machine, wherein an image on an original is read and input by image input means such as a scanner, the input image is subjected to a predetermined image process such as image-quality adjustment, edit processing, or compression/decompression, and the resultant image is output onto a paper sheet by image output means such as an electrophotographic printer.
Methods of compressing image data read by the scanner include an MH (modified Huffman) compression method and an MR (modified READ) compression method. In the MH compression, a length of continuous white pixels/black pixels, which is called a run length, is extracted from image data digitized in units of a scan line, and the extracted run length is encoded. The obtained code is a well-known variable-length code called a Huffman code. In the Huffman coding, a short code is assigned to more frequently occurring data, and a long code is to less frequently occurring data, whereby image data compression is effected. In the MR compression, correlation of adjacent scan lines is utilized on the basis of the MH compression, whereby coding efficiency is enhanced. There is known a processing method for effecting optimal data compression by selecting the MH compression or the MR compression.
There is also known a data compression method wherein even if the statistical properties of an information source are unknown, the data compression using the run length code can be performed by making use of the correlation of adjacent image data.
However, where a plurality of still image data of a plurality of originals input by the scanner are those of successive original images of the same format, as in the case of slips, data compression has to be effected in units of image data by the above-described method despite a great number of images of common parts being present. Since the images of common parts are compressed in units of image data, time is consumed uselessly and the amount of compressed data increases. Thus, the efficiency of compression is low.